Study: Fish Oil Does Not Help Prevent CancerFish oil is known to protect against heart disease, but new research finds fish oil plays no role in preventing cancer. Scientists are now investigating fish oil's role in shaping mood and memory.

Fish oil is known to protect against heart disease, but new research finds fish oil plays no role in preventing cancer. Scientists are now investigating fish oil's role in shaping mood and memory.

RENEE MONTAGNE, host:

This is MORNING EDITION from NPR News. I'm Renee Montagne. In our personal health segment, the science behind fish oils. The popular supplement is known to protect against heart disease. New research finds fish oil plays no role in preventing cancer. Researchers are now investigating fish oil's role in shaping mood and memory, as NPR's Allison Aubrey reports.

ALLISON AUBREY reporting:

Fish oils contain omega-three fatty acids, a type of fat the brain needs to function well, but can't produce efficiently on its own. So, when it comes to getting enough omega-threes, Cornell nutrition professor Tom Brenna says, there's only one way to do it.

Professor THOMAS BRENNA (Professor, Cornell University): We have to eat it, and we will eat it in the form of fish, or in the form of supplements, either fish oils or algae oils.

AUBREY: These are sources of the type of omega-three thought to be most beneficial; it's called DHA. The notion that fish oil is good for you goes way back. 85-year old Arthur Grado(ph) remembers his mother forcing teaspoons of vile tasting cod liver oil down his throat. In the 1920s the oil was known as a source of Vitamin D.

Mr. ARTHUR GRADO (Former fish oil consumer): Do I remember cod liver oil? I'll tell you how bad it got with my brother and I. When we knew it was time for cod liver oil, we would try to sneak underneath the bed.

AUBREY: But Grado says his mother would not let up.

Mr. GRADO: She wanted to make sure that there would be no bad germs entering our body, that it was for our health.

AUBREY: Grado found the explanation ridiculous, until decades later, when researchers began validating some of his mother's theories about fish oil.

Prof. BRENNA: This story is usually traced to classic studies by two fellows named Dyerberg and Bang, who went to Greenland and studied the natives there, and found that they had very low rates of cardiovascular disease, and that their diet consisted primarily of marine mammals.

AUBREY: And fish, from these early Eskimo studies in the 1970s. Lots of research followed. In one trial conducted in Italy, scientists supplemented people's diets with fish oil, and watched what happened to cardiovascular health over time. They documented a significant drop in heart attacks.

AUBREY: Tufts nutrition professor, Alice Lichtenstein says, what followed were findings that omega-threes contained both anti-inflammatory agents, and anti-clotting agents that cut the risk of strokes and blood clots.

Prof. LICHTENSTEIN: This evidence is quite solid.

AUBREY: And it led to hopes that perhaps omega-threes might also protect against cancer. A number of rat studies suggested that fish oil's anti-inflammatory effects could reduce the onset of cancer, and a few small scale human studies suggested the same. But when researchers combined data from dozens of nutrition studies that tracked people's consumption of fish, they found no link at all.

Professor MICHELLE HOLMS(Assistant Professor of Medicine, Harvard University): It just goes to show people aren't mice.

AUBREY: Harvard researcher Michelle Holmes says it's an important lesson in science. Theories and animal studies don't always hold up; testing takes time. When it comes to fish oil, the latest theory is that omega-threes play a role in regulating mood and memory. The theory stems from observations in babies' brains. Infants with high concentrations of the omega-three called DHA seem to have improved brain function.

Mr. BRENNA: You could say that DHA enables nerve communication to operate faster and more intense. A better way to say that is nerves communicate optimally when there's plenty of DHA around.

AUBREY: There are several clinical trials designed to measure the effect of Omega-threes on brain function and mood. Researchers at a VA hospital in New York are studying the correlation between fish oil and anger. And researchers in England are looking into depression. For now, nutritionist Alice Lichtenstein says people should think of fish oil as just one ingredient for healthy living.

Prof. LICHTENSTEIN: There's no one magic bullet that seems to decrease risk of all degenerative diseases. I think it's, what we're learning more and more is that it's the whole package.

AUBREY: Of diet and physical activity. As for concerns that fish oils may be tainted by environment pollution, such as mercury, Lichtenstein says, you'd have to eat inordinate amounts before the risk outweigh the cardiovascular benefits.

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